Srinivasan Chandrasekar

Low-cost manufacturing process for nanostructured metals and alloys

In spite of their interesting properties, nanostructured materials have found limited uses because of the cost of preparation and the limited range of materials that can be synthesized. It has been shown that most of these limitations can be overcome by subjecting a material to large-scale deformation, as occurs during common machining operations. The chips produced during lathe machining of a variety of pure metals, steels, and other alloys are shown to be nanostructured with grain (crystal) sizes between 100 and 800 nm. The hardness of the chips is found to be significantly greater than that of the bulk material.

Role of indentation fracture in free abrasive machining of ceramics

Abstract Free abrasive machining (FAM) is widely used for stock removal and surface finishing of ceramics. In FAM, material removal results from mechanical action between the abrasive slurry, which is trapped between the workpiece and a rotating lapping block, and the workpiece. Microscopic observations of the machined surface show that lateral cracking due to indentation by the abrasive particles contributes substantially to material removal. A simple model of FAM is developed which is based on indentation fracture and takes into account the abrasive particle distribution in the slurry. The model is used to predict the number of particles actually involved in the machining process, the distribution of load among these particles, and the depth of the plastically deformed layer on the workpiece surface. Many of the predictions of the model are well supported by experimental observations from the FAM of aluminum oxide, Ni-Zn ferrite, and glass using a silicon carbide slurry.

Sliding microindentation fracture of brittle materials: Role of elastic stress fields

Abstract An analytical model of the stress field caused by sliding microindentation of brittle materials is developed. The complete stress field is treated as the superposition of applied normal and tangential forces with a sliding blister approximation of the localized inelastic deformation occurring just underneath the indenter. It is shown that lateral cracking is produced by the sliding blister stress field and that median cracking is caused by the applied contact forces. The model is combined with measurements of the material displacement around an indentation to show that the relative magnitude of tensile stresses governing lateral crack and median crack growth varies with the magnitude of the applied load. The model also predicts a range of loads at which the lateral crack will grow only after the indenter is removed from the surface. These predictions are consistent with observations of the different regimes of cracking observed under a sliding pyramidal indenter in soda–lime glass and other brittle solids.

Machining as a Wedge Indentation

A case is made for the consideration of single-point machining of ductile metals as a special type of wedge indentation process. A general-purpose finite element analysis of machining using iterative rezoning is developed based on this analogy. The accuracy of this analysis, which does not incorporate any separation criterion, is limited only by our knowledge of the material properties and the friction conditions at the tool-chip interface. Strain hardening, strain rate effects, and the temperature dependence of the properties of the work material can be taken into consideration. While Coulomb friction is assumed at the chip-tool interface in the present model, it can easily be reformulated to include more complicated frictional interactions such as adhesion. An analysis of the cutting/ indentation of an isotropic work-hardening material at slow speeds under two different friction conditions is presented. It is shown that many of the important features of machining processes are consistently reproduced by the analysis.

Experimental and Computational Study of the Quenching of Carbon Steel

An investigation of the quenching of 1080 carbon steel cylinders has been carried out to determine the validity of a quenching process model for carbon steels. The process model included a description of the austenite-pearlite and austenite-martensite transformations in carbon steels, temperature-dependent material properties, and an elastic-plastic stress analysis. The model was simulated using the finite element method (FEM). An experimental study of the quenching of 1080 steel cylinders in water and two types of polymeric quenchants has also been carried out. The temperatures at three points within the cylinder during quenching were measured using thermocouples. The hardness and residual stress distributions along a cross-section of the quenched cylinders were determined using a Rockwell hardness test and an X-ray diffraction technique, respectively. The temperature-time histories, residual stress, and hardness distributions predicted from the FEM simulation of the quenching model were found to be in good agreement with the corresponding measurements. The quenching process simulation described in the study appears to be a promising tool for the design of heat-treatment process parameters for carbon steels.

Finite-element simulation of induction heat treatment

An efficient finite-element procedure has been developed for the analysis of induction heat treatment problems involving nonisothermal phase changes. The finite-element procedure first simulates the magnetic field developed when currents flow through an induction coil by solving Maxwell’s electromagnetic field equations; at the following step, it calculates the temperature distribution in the workpiece due to eddy currents induced by the magnetic field. The final stage of the simulation involves the determination of the distributions of residual stress, hardness, and microstructure in the workpiece. The finite-element analysis includes temperature-dependent material properties, changes in permeability of the workpiece at the Curie temperature, a mixed hardening rule to describe the material constitutive model, and the incorporation of time-temperature-transformation (TTT) diagram. The procedure was applied to the simulation of the induction hardening of 1080 steel bar. Firstly, the magnetic field and temperatures developed in the workpiece during (a) the induction heating of an infinitely long 1080 steel cylinder by a single encircling coil and (b) the induction heating of a semi-infinite half-space by a single coil suspended above it were calculated using the finite-element procedures. These were validated by comparing them with analytical solutions derived for these configurations using a Green’s function method. Finally, to demonstrate the predictive capability and practical applicability of the current finite-element procedure, two examples pertaining to the induction heat treatment of an infinite 1080 steel bar of square cross section and a notched finite 1080 steel cylinder of circular cross section were analyzed to predict the magnetic field, temperature, and residual stress distributions. The current finite-element procedure could be used as a powerful design tool for linking induction heat treating parameters with the mechanical property attributes of the heat treated component.

Measurement of Temperature Field in Surface Grinding Using Infra-Red (IR) Imaging System

An experimental technique is described for measuring the temperature field in a workpiece during surface grinding. The technique involves measurement of the radiation emitted by a side of the workpiece immediately adjoining the wheel-workpiece contact region using a Charge-Coupled Device (CCD) based Infra-Red imaging system. By using an appropriate calibration procedure, measured radiation values are converted to temperatures. Novel aspects of the experimental technique are full-field measurement of temperature at high spatial and temporal resolution, high sensitivity and non-intrusive measurement. The repeatability of temperature measurement is found to be very good. The experiments have provided an accurate estimate of the surface and sub-surface temperatures in the workpiece. Furthermore, by grinding along a taper with a continuously increasing depth of cut, the effect of material removal rate on temperature field has been characterized. Measurements of the temperature field in taper grinding have been found to correlate well with those made in conventional constant depth grinding thereby, establishing taper grinding as a viable, accelerated test for studying grinding temperatures. Full-field measurements of workpiece temperature should facilitate study of thermal damage and multi-scale validation of thermal models in grinding.

A New Approach for Studying Mechanical Properties of Thin Surface Layers Affected by Manufacturing Processes

Nano-indentation has become an important tool in the study of mechanical properties of solids at small length scales, ever since its formulation as a technique in the early 1980s.* The small size of an indentation, typically one micrometer or less in surface extent, makes it a potentially attractive tool also for the quantitative study of the characteristics of surface layers in monolithic solids. Here, we report results from a study in which nano-indentation has been combined with taper-sectioning to analyze the mechanical properties of thin surface layers affected by manufacturing processes. The so-called white etching layer (WL) produced in steel surfaces by machining and abrasion is characterized. The WL is found to have a hardness in the range of 11.5-16.2 GPa, which is significantly greater than that of untempered martensite produced by heat treatment processes. These hardness values are close to those measured on steel piano wires. The so-called burn-layer produced on ground surfaces of steels is found to have a hardness distribution very similar to that of a white layer suggesting that the two layers are of the same type. Localized hardening and softening of surface layers, over spatial extents of a few micrometers, caused by material removal processes are accurately resolved by the combined use of nano-indentation and taper-sectioning. The taper-sectioning/nano-indentation approach is also shown to be a good procedure for characterizing the hardness of PVD-TiN films deposited onto hard metal substrates.

Stabilizing nanostructured materials by coherent nanotwins and their grain boundary triple junction drag

The role of nanotwin lamellae in enhancing thermal stability of nanostructured materials is examined. Nanostructured copper with varying densities of twins was generated by controlling the deformation strain rate during severe plastic deformation at cryogenic temperatures. While the nanostructured materials produced under cryogenic conditions are characteristically unstable even at room temperatures, their stability is markedly improved when a dense dispersion of nanotwins is introduced. Observations of the role of nanotwins in pinning grain and subgrain structures suggest an interfacial engineering approach to enhancing the stability of nanostructured alloys.

On two indentation hardness definitions

The difference between the contact area under load and the residual projected area of an indentation after complete unloading has been studied for various materials using finite element and dimensional analyses of cone indentation. The difference in the contact areas gives rise to two hardness values, one based on the contact area under load and another based upon the residual projected area. The effect of sinking-in, piling-up and elastic recovery on the residual contact area has been calculated for materials with different elastic properties and strain hardening characteristics. Based on this study, an estimation procedure is suggested for obtaining a more accurate value of the hardness under load from a measurement of the residual area of an indentation.